1. Field of the Invention
The present invention relates to an organic light emitting device (OLED) and a method for manufacturing the same.
2. Discussion of the Related Art
The OLED uses a phenomenon that an electron and a hole form an electron-hole pair in a semiconductor, or carriers are excited to a higher energy state and then fall down to a ground state, which is a stable state, and emit light. Since the OLED is a self-luminous type device as described above, it does not require a separate backlight unit unlike a liquid crystal display (LCD) device. Accordingly, the OLED can be manufactured in a lightweight, slim profile, and the OLED display has low power consumption and excellent viewing angle and contrast. Also, the OLED can be driven at a low DC voltage and has a fast response time. Also, since the OLED is in a solid state, it is resistant to impact, can be used over a wide range of temperature, and particularly, can be manufactured at low costs.
The OLED is classified into a passive matrix OLED and an active matrix OLED depending on a driving method.
Since the passive matrix OLED has a simple construction, a manufacturing method thereof is also simple, but the passive matrix OLED has high power consumption and is difficult to manufacture in a large size. Also, when the number of lines increases, an aperture ratio decreases.
On the other hand, the active matrix OLED can provide high light emitting efficiency and high image quality.
FIG. 1 is a schematic cross-sectional view of a related art OLED.
As illustrated in FIG. 1, the related art OLED 10 includes a thin film transistor array 14 on a first substrate 12, a first electrode 16 independently patterned for each pixel and an organic light emitting layer 18 on the thin film transistor array 14, a second electrode 20 formed on an entire surface of the substrate including the organic light emitting layer 18.
The light emitting layer 18 expresses red (R), green (G), and blue (B) colors. Generally, separate organic materials emitting light of R, G, and B colors are patterned for respective pixels P.
The first substrate 12 is attached to a second substrate 28 on which a moisture absorbent 22 is attached using a sealant 26, so that the OLED is completed.
The moisture absorbent 22 is designed for removing moisture that can penetrate into a capsule. The moisture absorbent 22 is fixed by etching a portion of the substrate 28, disposing the powder type moisture absorbent 22 into the etched portion, and attaching a tape 25.
The above-described OLED is a bottom emission OLED. The bottom emission OLED has stability by a sealing process and high degree of freedom in processing, but has a limited aperture ratio and thus is difficult to apply to a high resolution product.
On the other hand, in a top emission OLED, a thin film transistor is easily designed and the aperture ratio can be improved, so that the top emission OLED is advantageous with respect to product life.
In a related art top emission OLED, the first electrode 16 is used as a cathode, and the second electrode 20 is used as an anode. Since the first electrode 16 is connected to a drain electrode of a driving thin film transistor and is formed on the driving thin film transistor to improve an aperture ratio, the first electrode 16 has an influence on an output current saturation characteristic of the thin film transistor T.
Here, an output current means a current generated at a portion of a semiconductor layer that is located between a source electrode and a drain electrode depending on a voltage change of a gate electrode.
That is, a cathode connected to the drain electrode in the driving thin film transistor is formed on a channel of the driving thin film transistor. A voltage of the cathode behind the channel changes as a voltage of the drain electrode changes. When the voltage of the cathode on the channel changes, a field effect is generated in a portion of the channel that is located under the cathode, so that the current through the channel increases. Also, a field effect is generated in the channel as a voltage of the cathode changes, which may result in a continuous increase of an output current, instead of the saturation. The continuous increase of the output current may affect the display brightness, as opposed to the saturated output current which may not change the display brightness. Accordingly, a gray black point on an OLED or non-uniformity in brightness may occur, so that image quality deteriorates.